Spin labeling segmental mobility

The structural analysis on KcsA was performed based on the mobility of each spin labeled side chain in the protein segments under investigation. It is worth recognizing in Pig. 4b that most of the CW RT spectra show multiple spectral components, characterized by different mobility (a few examples are highlighted by arrows). This is a very general property of the R1 side chain in proteins. The components reflect the anisotropy of the spin label reorientational motion, but their appearance could also have other causes. They could arise from a slow equilibrium between two different protein conformations or the presence of asymmetric sites in the protein. The molecular interpretation of different spectral components is cumbersome. Multifrequency EPR [17], temperature analysis of the CW spectra [27], pulse saturation recovery techniques [28], or high pressure EPR [29] can help unravel the possible origins of the spectral components. In the case of KcsA, the spin labels motional information was quantitatively extracted from the inverse central line width (A//q, mobility parameter) and was corroborated by the measure of the accessibility of the spin labeled side chains towards lipids (O2... 

Abstract Measurements of temperature dependent ESR spectra of spin labels trapped in the polymer matrices is very effective for the evaluation of molecular mobility (molecular motion) of polymer chains. We can characterize the molecular motion of the particular sites in different region by the ESR method. It is possible to detect the mobility of polymer material at the segment or atomic level. ESR studies help to relate the features of the nanometer scale to macroscopic properties of the polymer materials. We present examples of spin labeling studies in the polymer science to help readers new to the field understand how and for what areas the method is effective. In the first part, we give a simple review of the spin labeling method and present applications in the polymer physics related to relaxation phenomena in various systems. In the second part applications to biopolymer system are introduced to help the clarification of various mechanisms of bio-membranes. 

Miwa Y, Drews AR, Schlick S (2008) Unique structure and dynamics of poly(ethylene oxide) in layered silicate nanocomposites accelerated segmental mobility revealed by simulating ESR spectra of spin-labels, XRD, FTIR, and DSC. Macromolecules 41 4701-4708 Mohan SD, Mitchell GR, Davis FJ (2011) Chain extension in electrospun polystyrene fibres a SANS study. Soft Matter 7 4397-4404... 

Electron spin resonance (ESR) spectroscopy of labeled polymers can be employed to determine the mobility of a polymer molecule in a blend. The polymer can be covalently bonded with a spin label (nitroxyl radical) or a spin probe can be blended in the polymer blend matrix. This method allows for observation of segmental motions at a length scale smaller than that typically observed with the glass transition. The ESR spectra depends on the environment of the spin labels or probes and can be used to assess phase behavior. A review of ESR spectroscopy applied to polymer blends can be found in [416]. 

The ESR difference spectra of the motionally restricted lipid component (cf. Fig. 3.2) lie within the slow-motion regime of spin label spectroscopy, indicating that the effective rotational correlation times are 10 ns. Experiments on the temperature dependence in rod outer segment disc membranes show changes which are diagnostic for motion on this timescale (Watts et al., 1981 1982). Thus the mobility of the motionally restricted lipid is significantly reduced relative to that of the fluid lipids ( ns), but... 

Watts, A., Davoust, J., Marsh, D., and Devaux, P.F., 1981, Distinct states of lipid mobility in bovine rod outer segment disc membranes. Resolution of spin label results, Biochim. Biophys. Acta, 643 673. 

Spin, Mossbauer, fluorescent and phosphorescent labels were introduced into the various portions of the system being studied. They were covalently bound to the RC surface groups, adsorbed by the hydrophobic segments of the protein and membrane, and 57Fe atoms were incorporated by way of biosynthesis into iron-containing proteins. Then, in the same samples, the dependence on temperature, moisture content and viscosity was measured for the label mobility and the rate constant of electron transfer... 

Electron spin resonance (5) measures the difference in the relaxation process between the segments in the loops and trains. The spectra of an adsorbed layer is deconvoluted into contributions from the loops, the tails, and the trains of the adsorbed layer by comparison with the spectra obtained in solution or melt state (mobile components) and on adsorbed oUgomers at low temperatures (immobile components). It is also possible to measure the rotational correlation time of the mobile and immobile segments. The drawback of this technique is that a label has to be attached to the polymer and this might change the adsorption behavior of the chain. 

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